1. Field of the Invention
This invention relates to devices which receive light by optically pumping, and more specifically, to optically pumped laser devices.
2. Description of the Related Art
Optically pumped semiconductor lasers that emit radiation at wavelengths greater than 3xc3x9710xe2x88x926 meters (3 m) are needed for a variety of applications, including chemical sensing, environmental monitoring, and infrared countermeasures. Currently, lasers in this spectral region are inefficient, and high power operation generally requires cryogenic cooling. Operation of these lasers at higher temperatures which would allow the use of thermoelectric cooling rather then cryogenic cooling systems is especially desirable. To date, the only III-V mid-infrared (IR) laser class, either interband or intersubband, to have achieved continuous wave lasing at temperatures greater than 210 K is the optically pumped type II xe2x80x9cWxe2x80x9d structure. By using a diamond-pressure-bond (DPB) technique, near room temperature (290 K) continuous wave (cw) operation has been obtained for a xe2x80x9cWxe2x80x9d laser emitting at =3 m, according to xe2x80x9cHigh-Temperature Continuous Wave 3-6.1 m xe2x80x9cWxe2x80x9d Lasers with Diamond-Pressure-Bond Heat Sinkingxe2x80x9d, by W. W. Bewley, C. L. Felix, I. Vurgaftman, D. W. Stokes, E. A. Aifer, L. J. Olafsen, and J. R. Meyers, Applied Physics Letters 74 (8), 1075 (1999). However, at T=180K the continuous wave maximum power Pmax was approximately equal to 40 mW per facet, and decreased rapidly beyond that temperature due to poor efficiency caused by strong intervalence absorption. It is clear that internal losses and thresholds need to be reduced and efficiencies increased if there are to be any large additional enhancements of the continuous wave output powers at either low or high temperatures, since diamond pressure bond mounting has already improved the thermal management to the point where it represents a secondary rather than primary performance limitation.
Optically pumped type II mid-IR (3-5 xcexcm) semiconductor lasers are usually pumped by a diode or solid state laser with a pump wavelength in the approximate range of 1-2.5 xcexcm. Typical pump sources are a Nd:YAG laser or InGaAsSb/AlGaAsSb diode lasers.
Most optically pumped semiconductor lasers consist of an active region that produces the optical gain at the lasing wavelength, which is surrounded top and bottom by an optical cladding material, and grown on a substrate such as GaSb. The pump radiation can be incident either from the top or from the bottom (through the substrate). For incidence from the top, some fraction of the incident radiation is absorbed on its first pass through the active region. However, the remainder is transmitted to the substrate, after which some fraction of the transmitted portion may be reflected back through the optical cladding and into the active region for a second pass.
Many high power cw and quasi-cw optically pumped mid-IR lasers have tried using a short pump wavelength xcexp (e.g. of approximately 1 xcexcm), because of its large absorption coefficient in the active region. In combination with a thick active region [e.g. 50-100 quantum wells or a thick double heterostructure (DHS) region] over 80% of the pump beam can be absorbed in the active layer. A disadvantage of short wavelength pumping is that the large photon decrement (e.g. 3-5:1 in typical mid-IR lasers) limits the maximum efficiency and causes most of the absorbed radiation to be converted into heat. Moreover, free carrier absorption losses, which can significantly degrade laser efficiency, are roughly proportional to the total number of carriers present in the cavity, which are typically high for a thick active region. These losses may be reduced by employing fewer quantum wells or a thinner DHS active region, a trade-off that leads to a weakening of the pump-beam absorbance, so that lower rather than higher efficiencies generally result.
Another approach has been to pump high power cw and quasi-cw optically pumped lasers at a longer pump wavelength, e.g. 1.8-2 xcexcm, which has several advantages. Use of the longer pump wavelength improves the photon decrement, and therefore, increases the theoretical limit to the efficiency. Because the substrate, for the example of mid-IR lasers, is typically GaSb, and GaSb is transparent to 1.8-2 xcexcm radiation, better heat removal is possible by mounting the laser epilayer-side-down (where the pump beam is incident on the GaSb substrate). The lower photon decrement and the lower absorption in the substrate typically allow operation of a 1.8-2 xcexcm pumped laser at a higher temperature than a 1 xcexcm pumped mid-IR laser.
The drawback of pumping a mid-IR laser at 1.8-2 xcexcm is that absorption of the longer wavelength pump beam is considerably weaker, so that typically only 10-20% will be absorbed in a single pass through the active region. While a second pass through the active region can follow reflection from a metallization layer or heat sink, it is still difficult to absorb more than 15-35% of the pump beam.
Another approach that has been tried in the mid-IR to improve on the weak pump-beam absorption at xcexp=1.8-2 xcexcm is to add more quantum wells to the active region. However, this strategy can become self defeating, as it leads to increased internal losses at high temperatures. Additionally, more heat is generated within the active region and the thermal resistance to heat extraction significantly increases. In fact, less than ten quantum wells would be optimal from a thermal management standpoint. Devices pumped at 1.8-2 xcexcm can also be grown with extra regions surrounding the quantum wells to absorb more of the pump radiation and allow the generated electron hole pairs to diffuse into the quantum wells. This integrated absorber strategy is limited by weaker absorption in these extra regions than in the active region. While to date, it has had some success, it entails the difficult growth of quaternary regions or short periods superlattices with poor thermal transport properties. Additional carrier confinement issues related to potentially inadequate valence band offset can also arise. It should be noted that the present invention is fully compatible with the integrated absorber approach, so that a combination of the two may be advantageous for some applications.
Ideally, a mid-IR optically pumped laser would combine a high pump absorbance with long pump wavelength and a thin active layer. An optical pumping injection cavity (OPIC) approach, employing a Fabry Perot etalon cavity tuned to the pump wavelength, can provide all three of these features.
It is an object of the invention to improve the efficiency of lasers that are optically pumped by a narrow bandwidth source that is incompletely absorbed by a single or double pass through the laser active region.
It is an object of the invention to improve the absorption of the pump energy in the active region of an optically pumped laser.
It is an object of the invention to lower the threshold pump intensity of an optically pumped laser.
It is another object of the invention to reduce the amount of cooling needed to operate a high power optically pumped laser.
It is another object of the invention to increase the temperature at which a high power optically pumped laser can operate.
It is another object of the invention to provide low cost, high power, optically pumped edge emitting and surface emitting lasers which can be operated at elevated temperatures or power levels.
It is an object of the invention to provide high power optically pumped semiconductor lasers with high pump absorbance, long pump wavelength, and a thin active layer.
It is another object of the invention to provide lasers with a small number of quantum wells in the active region optimized for pumping at a longer wavelength with high absorption of the pump energy.
The advantages of the OPIC approach have been demonstrated to be especially advantageous in mid-IR (3-5 xcexcm) semiconductor lasers fabricated on GaSb substrates. However, the approach is quite general to other substrates, active layers, and emission wavelengths. It may also be advantageous when applied to optically-pumped lasers whose active regions are not semiconductors.
It is another object of the invention to provide an optical device which can efficiently convert pump radiation to absorbed energy in an optical gain medium, through multiple passes of the pump radiation through the active region.
In accordance with these and other objects made apparent herein, the invention concerns an apparatus for receiving optical pump radiation having a wavelength xcexp and transmitting pump light to an active region. The apparatus includes a first and second reflector, each reflector being reflective at the wavelength xcexp, disposed on opposite sides of the active region, where the separation between the reflectors creates a cavity that is resonant at the pump wavelength xcexp. The distance is preferably about mxcexp/2n cos xcex8, where n is the average index of refraction of the material having an active region, xcex8 is the angle that the pump beam transverses relative the normal of said reflectors, and m is any positive integer. In some configurations of the invention, some or all of the active gain medium may be contained within the first and/or second reflector, in which case m may have the value zero in addition to all positive integers.
These and other objects, features, and advantages of the invention will become better understood by reference to the following detailed description.